Ion exchangeable li-containing glass compositions for 3-d forming

ABSTRACT

According to one embodiment, a glass article may include SiO 2 , Al 2 O 3 , Li 2 O and Na 2 O. The glass article may have a softening point less than or equal to about 810° C. The glass article may also have a high temperature CTE less than or equal to about 27×10 −6 /° C. The glass article may also be ion exchangeable such that the glass has a compressive stress greater than or equal to about 600 MPa and a depth of layer greater than or equal to about 25 μm after ion exchange in a salt bath comprising KNO 3  at a temperature in a range from about 390° C. to about 450° C. for less than or equal to approximately 15 hours.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No. 13/938579 filed on Jul. 10, 2013, which in turn claims the benefit of priority under 35 U.S.C. § 119 of U.S. Provisional Patent Application Ser. No. 61/672,346 filed Jul. 17, 2012 and entitled “Ion Exchangeable Li-Containing Glass Compositions For 3-D Forming,” the entirety of each is hereby incorporated by reference.

BACKGROUND Field

The present specification generally relates to glass compositions suitable for use in 3-D forming applications and, more specifically, to ion exchangeable, Li-containing glass compositions suitable for 3-D forming.

Technical Background

Ion exchangeable glass compositions are widely used as cover glasses in many electronic devices including mobile telephones, personal media players, tablet computers and the like. The cover glasses used in these applications are generally flat and planar. As such, the cover glasses may be formed using conventional glass forming processes, such as down draw processes and/or float processes.

One limiting factor in the aesthetic design of electronic devices is the ability to shape the cover glasses to conform to curved and/or complex contours. Glass compositions which are amenable to ion exchange generally have relatively high softening points, making the glass compositions difficult to form into 3-D shapes using elevated temperature forming processes such as vacuum sagging. As a result of the relatively high softening points, the glass compositions tend to react with the material of the mold, sticking to the mold and/or degrading the mold, even when protective coatings are applied to the mold.

Accordingly, a need exists for alternative glass compositions suitable for use in elevated temperature 3-D forming processes and which are also amenable to strengthening by ion exchange processing.

SUMMARY

According to one embodiment, a glass article may include SiO₂, Al₂O₃, Li₂O and Na₂O. The glass article may have a softening point less than or equal to about 810° C. The glass article may also have a high temperature coefficient thermal expansion (“CTE”) less than or equal to about 27×10⁻⁶/° C. The glass article may also be ion exchangeable such that the glass has a compressive stress greater than or equal to about 600 MPa and a depth of layer greater than or equal to about 25 μm after ion exchange in a salt bath comprising KNO₃ at about 410° C. in a temperature range from about 390° C. to about 450° C. for less than or equal to approximately 15 hours.

In another embodiment, a glass composition may include from about 65.8 mol. % to about 71 mol. % SiO₂; from about 7 mol. % to about 12 mol. % Al₂O₃; from about 1 mol. % to about 9 mol. % Li₂O; from about 6 mol. % to about 16 mol. % Na₂O; and from about 0.8-10 mol. % of a divalent oxide, wherein the divalent oxide includes at least one of MgO and ZnO; and less than about 0.5 mol. % B₂O₃. A sum of a concentration of Al₂O₃ (mol. %) and a concentration of the divalent oxide (mol. %) may be greater than 10 mol %. The glass composition may have a softening point less than or equal to about 810° C. The glass composition may also have a high temperature coefficient thermal expansion (“CTE”) less than or equal to about 27×10⁻⁶/° C. These glass compositions may be substantially free of ZrO₂.

In yet another embodiment, a glass composition may include from about 55 mol. % to about 68 mol. % SiO₂; from about 9 mol. % to about 15 mol. % Al₂O₃; from about 4.5 mol % to about 12 mol. % B₂O₃; from about 1 mol. % to about 7 mol. % Li₂O; from about 3 mol. % to about 12 mol. % Na₂O; and from about 0 mol. % to about 3 mol. % K₂O. In this embodiment, R₂O is a sum of a concentration of Li₂O, a concentration of Na₂O, and a concentration of K₂O. The ratio of R₂O to a concentration of Al₂O₃ is less than or equal to about 1.5. The glass composition may have a softening point less than or equal to about 810° C. The glass composition may also have a high temperature CTE less than or equal to about 27×10⁻⁶/° C.

In yet another embodiment, a glass composition may include from about 65 mol. % to about 71 mol. % SiO₂; from about 7 mol. % to about 12 mol. % Al₂O₃; from about 1 mol. % to about 9 mol. % Li₂O; from about 6 mol. % to about 16 mol. % Na₂O; from about 0 mol. % to about 5 mol. % K₂O; from about 0.8 to about 10 mol. % of a divalent oxide, wherein the divalent oxide comprises at least one of MgO and ZnO; from about 0.5 mol. % to about 2 mol. % ZrO₂; and less than about 0.5 mol. % B₂O₃, wherein: the sum of the concentration of Al₂O₃ (mol. %), and the concentration of the divalent oxide (mol. %) is greater than about 10 mol %. The glass composition has a softening point less than or equal to about 810° C.; and a high temperature CTE less than or equal to about 27×10⁻⁶/° C.

Additional features and advantages of the glass compositions described herein will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the embodiments described herein, including the detailed description which follows, the claims, as well as the appended drawings.

It is to be understood that both the foregoing general description and the following detailed description describe various embodiments and are intended to provide an overview or framework for understanding the nature and character of the claimed subject matter. The accompanying drawings are included to provide a further understanding of the various embodiments, and are incorporated into and constitute a part of this specification. The drawings illustrate the various embodiments described herein, and together with the description serve to explain the principles and operations of the claimed subject matter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 graphically depicts the instantaneous CTE (y-axis) as a function of temperature (x-axis) for two comparative glass compositions;

FIG. 2 graphically depicts the softening point (y-axis) as a function of the concentration of Li₂O (x-axis) as Li₂O is substituted for Na₂O in an exemplary glass composition;

FIG. 3 graphically depicts the HT CTE (y-axis) as a function of the concentration of Li₂O (x-axis) as Li₂O is substituted for Na₂O in an exemplary glass composition;

FIG. 4 graphically depicts the softening point (y-axis) as a function of the concentration of Li₂O (x-axis) as Li₂O is substituted for Na₂O and K₂O in an exemplary glass composition;

FIG. 5 graphically depicts the HT CTE (y-axis) as a function of the concentration of Li₂O (x-axis) as Li₂O is substituted for Na₂O and K₂O in an exemplary glass composition;

FIG. 6 graphically depicts the compressive stress and DOL values plotted for different Li₂O concentrations; and

FIG. 7 graphically depicts the concentration of potassium and sodium ions (y-axis) as a function of depth (x-axis) for an ion-exchanged glass plate formed from an exemplary glass composition containing color modifying constituents.

DETAILED DESCRIPTION

Reference will now be made in detail to various embodiments of ion exchangeable glass compositions which are suitable for use with 3-D forming processes. The glass compositions described herein generally include SiO₂, Al₂O₃, Li₂O and Na₂O. The glass composition may have softening points less than or equal to about 810° C. The glass composition may also have high temperature CTEs less than or equal to about 27×10⁻⁶/° C. The glass compositions may also be ion exchangeable such that the glass has a compressive stress greater than or equal to about 650 MPa and a depth of layer greater than or equal to about 25 μm after ion exchange in a salt bath comprising KNO₃ at about 410° C. for less than or equal to approximately 15 hours. Various embodiments of the glass compositions will be described in further detail herein with specific reference to the appended drawings.

The term “softening point,” as used herein, refers to the temperature at which the viscosity of the glass composition is 1×10^(7.6) poise.

The phrase “high temperature coefficient of thermal expansion” or “HT CTE,” as used herein, refers to the coefficient of thermal expansion of the glass composition above the glass transition temperature of the glass composition. The HT CTE is determined by plotting the instantaneous CTE (y-axis) as a function of the temperature (x-axis). The HT CTE is the value of the HT CTE where the slope of the CTE v. temperature curve is approximately zero following a pronounced increase (i.e., where the CTE v. temperature curve “plateaus”). The value of the HT CTE is a measure of the volume change of the glass during cooling and is an indication of the dimensional stability of the glass composition when the glass is utilized in conjunction with elevated temperature 3-D forming process including, without limitation, vacuum sagging forming processes.

The term “liquidus viscosity,” as used herein, refers to the shear viscosity of the glass composition at its liquidus temperature.

The term “liquidus temperature,” as used herein, refers to the highest temperature at which devitrification occurs in the glass composition.

The term “substantially free,” when used to described the absence of a particular oxide component in a glass composition, means that the component is present in the glass composition as a contaminant in a trace amount of less than about 0.05 mol. %.

In the embodiments of the glass compositions described herein, the concentration of constituent components (e.g., SiO₂, Al₂O₃, B₂O₃ and the like) are given in mole percent (mol. %) on an oxide basis, unless otherwise specified.

Conventional ion exchangeable glass compositions used as cover glasses in consumer electronic devices, generally have softening points of 840° C. or greater. Glasses with softening points in this range may be readily formed into planar sheets using fusion forming processes. However, such glass compositions are not always amenable to elevated temperature forming process. Specifically, the relatively high softening points of the glass compositions cause the glass compositions to react with the material of the mold such that the glass composition either sticks to the mold damaging the glass and/or degrading the mold, even when protective coatings are applied to the mold.

Further, attempts to improve the formability of ion exchangeable glass compositions by decreasing the softening points of the glass compositions have not been successful. Specifically, it has been found that glass compositions with lower softening points did not have the requisite dimensional stability for 3-D forming using elevated temperature processes such as vacuum sagging. Such glass compositions warp upon forming as the compositions are heated and/or cooled through the glass transformation region.

For example, FIG. 1 graphically depicts the instantaneous CTE (y-axis) as a function of temperature (x-axis) for two comparative glass compositions. Comparative glass A was a borosilicate glass which had a softening point of 752° C. and an HT CTE of approximately 39×10⁻⁶/° C. While not wishing to be bound by theory, it is believed that this relatively high HT CTE reduced the dimensional stability of the glass upon vacuum sagging, causing the glass to warp and distort. In contrast, Comparative glass B was an aluminosilicate glass which had a softening point of 837° C. and an HT CTE of approximately 23.2×10⁻⁶/° C. While this glass exhibited a relatively low HT CTE, it was found that the glass composition reacted with and/or stuck to the mold during vacuum sagging, inhibiting formation. While not wishing to be bound by theory, it is believed that the inability to consistently form Comparative glass B was due, at least in part, to the relatively higher softening point of the glass.

The glass compositions described herein address the deficiencies of previous glass compositions by providing glass compositions which have relatively low softening points, relatively low HT CTEs and have superior ion exchange performance relative to existing 3-D formable glass compositions.

In the embodiments described herein, the glass compositions have relatively low softening points of less than or equal to about 810° C. In some embodiments, the softening point of the glass composition may be less than or equal to about 800° C. or even less than or equal to about 790° C. In some other embodiments, the softening points may be less than about 750° C. The relatively low softening points of these glass compositions facilitate readily forming the glass compositions into 3-D shapes, such as glass articles with complex curvatures and the like, using vacuum sagging processes.

The glass compositions also have HT CTEs of less than or equal to about 27×10⁻⁶/° C. In some embodiments, the HT CTE of the glass composition may be less than or equal to about 25×10⁻⁶/° C. or even less than or equal to about 23×10⁻⁶/° C. As noted above, the HT CTE is an indication of the dimensional stability of the glass when the glass is utilized in conjunction with elevated temperature 3-D forming process including, without limitation, vacuum sagging processes. It has been determined that glasses which have HT CTEs greater than 27×10⁻⁶/° C. may warp during and/or after elevated temperature forming processes resulting in a glass article which may not conform to dimensional tolerances. However, it has also been determined that glasses with moderately lower HT CTEs, such as HT CTEs less than or equal to about 27×10⁻⁶/° C. are dimensionally stable during and following elevated temperature forming processes.

The glass compositions described herein are also amenable to strengthening by ion exchange processes. In the embodiments described herein, the glass compositions are able to achieve a depth of layer (DOL) of greater than or equal to about 25 μm. In some embodiments, the DOL may be greater than or equal to about 35 μm or even greater than or equal to about 45 μm. The compressive stress (CS) of the glass composition may be greater than or equal to about 600 MPa or even greater than or equal to about 650 MPa. Both the compressive stress and the DOL are determined following ion exchange strengthening in a salt bath comprising 100% KNO₃ or a salt bath comprising KNO₃ and NaNO₃ for less than or equal to approximately 15 hours at temperatures from about 390° C. to about 450° C.

In order to achieve the aforementioned properties, the glass compositions described herein generally include a combination of SiO₂, Al₂O₃, and alkali oxides such as Li₂O and/or Na₂O. In some embodiments, the glass compositions may also include one or more divalent oxides, such as MgO, ZnO, CaO or the like. The glass compositions may also include B₂O₃. In some embodiments, the glass compositions may also comprise K₂O in addition to Li₂O and/or Na₂O. The glass compositions may additionally comprise P₂O₅. The glass compositions may also comprise one or more fining agents. The concentrations of these various constituent components used to achieve glass compositions having the aforementioned properties will be described in further detail herein.

As noted above the glass compositions described herein may include B₂O₃. In some embodiments, the concentration of B₂O₃ in the glass composition may be less than or equal to about 1.0 mol. % or even less than or equal to about 0.5 mol. %, including about 0 mol. % (i.e., glass compositions which are substantially free from B₂O₃). These glass compositions may be referred to herein as “low boron glass compositions.” In other embodiments, the concentration of B₂O₃ may be greater than or equal to about 4.5 mol. %. These glass compositions may be referred to herein as “high boron glass compositions.” However, it should be understood that the low boron glass compositions and the high boron glass compositions both exhibit the relatively low softening points, relatively low HT CTEs and ion exchangeability described above.

In the embodiments of the glass compositions described herein (i.e., both low boron glass compositions and high boron glass compositions), SiO₂ is the largest constituent of the composition and, as such, SiO₂ is the primary constituent of the glass network. When the concentration of SiO₂ in the glass composition is low (i.e., less than about 55 mol. %) the chemical durability of the resultant glass is low. In addition, the liquidus viscosity of the resultant glass may also be low rendering the glass unsuitable for fusion formation, such as with a fusion down draw process and/or a fusion lamination process. However, if the concentration of SiO₂ in the glass composition is too high (i.e., greater than about 75 mol. %), the formability of the glass composition may be diminished as higher concentrations of SiO₂ increase the difficulty of melting the glass which, in turn, adversely impacts the formability of the glass. In the embodiments described herein, the glass composition generally comprises SiO₂ in a concentration greater than or equal to about 55 mol. % and less than or equal to about 75 mol. % in order to facilitate a readily formable glass compositions.

In the low boron glass compositions, the concentration of SiO₂ may be greater than or equal to about 65 mol. % and less than or equal to about 71 mol %. In some embodiments, the concentration of SiO₂ may be greater than or equal to about 65.8 mol. % or even about 66 mol. % and less than or equal to about 71 mol. %. In some other embodiments, the concentration of SiO₂ in the glass composition may be greater than or equal to about 67 mol. % and less than or equal to about 71 mol. %. In some embodiments, the concentration of SiO₂ in the glass composition may be greater than or equal to about 68 mol. % and less than or equal to about 71 mol. %.

In the high boron glass compositions, the concentration of SiO₂ may be greater than or equal to about 55 mol. % and less than or equal to about 68 mol. %. In some of these embodiments, the concentration of SiO₂ in the glass composition may be greater than or equal to about 60 mol. % and less than or equal to about 65 mol. %.

The glass compositions described herein (i.e., both low boron glass compositions and high boron glass compositions) also comprise Al₂O₃. Al₂O₃ serves as a glass network former, similar to SiO₂. Like SiO₂, Al₂O₃ increases the viscosity of the glass composition due to its primarily tetrahedral coordination in a glass melt formed from the glass composition. Al₂O₃ improves the ion-exchange performance of the glass composition by increasing the strain point of the glass and increasing the diffusivity of alkali ions in the glass network. Accordingly, the presence of Al₂O₃ improves the kinetics of the ion-exchange process and increases the maximum compressive stress and DOL which can be obtained. In order to obtain the improvement in the kinetics of the ion-exchange process, the concentration of Al₂O₃ in the glass compositions is generally greater than or equal to about 7 mol. %.

In the embodiments of the low boron glass compositions described herein, the concentration of Al₂O₃ in the glass compositions is generally less than or equal to about 12 mol. % in order to achieve a glass composition which has a relatively low softening point. For example, in some embodiments, the concentration of Al₂O₃ in the glass compositions is greater than or equal to about 7 mol. % and less than or equal to about 12 mol. %. In some embodiments, the concentration of Al₂O₃ in the glass compositions may be greater than or equal to about 8 mol. % and less than or equal to about 12 mol. %. In some other embodiments, the concentration of Al₂O₃ may be greater than or equal to about 8 mol. % and less than or equal to about 11 mol. %.

In the embodiments of the high boron glass compositions described herein, the concentration of Al₂O₃ in the glass compositions is generally less than or equal to about 15 mol. % in order to achieve a glass composition which has a relatively low softening point. For example, in some embodiments, the concentration of Al₂O₃ in the glass compositions is greater than or equal to about 9 mol. % and less than or equal to about 15 mol. %. In some embodiments, the concentration of Al₂O₃ in the glass compositions may be greater than or equal to about 11 mol. % and less than or equal to about 14 mol. %.

The glass compositions described herein (i.e., both low boron glass boron glass compositions and high boron glass compositions) also include alkali oxide R₂O where R is at least one of Li, Na, K or combinations thereof. The alkali oxides lower the melting temperature and the liquidus temperature of the glass, thereby improving the formability of the glass composition. Additions of Li₂O generally decrease the softening point of the glass. The amount of Li₂O in the glass composition can be adjusted to improve the reaction kinetics of the ion exchange process. Specifically, if a faster ion exchange process is desired, the concentration of Li₂O in the glass compositions may be optionally decreased to less than about 5 mol. %, such as from greater than or equal to about 1 mol. % to less than or equal to about 5 mol. %, or even from greater than or equal to about 2 mol. % to less than or equal to about 5 mol. %, in order to increase the ion exchange rate while also decreasing the softening point of the glass.

In the embodiments of the low boron glass compositions described herein, Li₂O is generally added to the glass compositions to decrease the softening point of the glass. The concentration of Li₂O in the glass compositions is generally greater than or equal to about 1 mol. % in order to achieve a glass composition which has a relatively low softening point. For example, in some embodiments, the concentration of Li₂O in the glass compositions is greater than or equal to about 1 mol. % and less than or equal to about 9 mol. %. In some embodiments, the concentration of Li₂O in the glass compositions may be greater than or equal to about 1 mol. % and less than or equal to about 7 mol. %. In embodiments where a faster ion exchange time is desired for a given temperature, the concentration of Li₂O may be greater than or equal to about 1 mol. % and less than or equal to about 5 mol. %, or even greater than or equal to about 2 mol. % and less than or equal to about 5 mol. %. In some other embodiments, the concentration of Li₂O may be greater than or equal to about 2 mol. % and less than or equal to about 3.5 mol. %.

In the embodiments of the high boron glass compositions described herein, the concentration of Li₂O in the glass compositions is generally greater than or equal to about 1 mol. % in order to achieve a glass composition which has a relatively low softening point. For example, in some embodiments, the concentration of Li₂O in the glass compositions is greater than or equal to about 1 mol. % and less than or equal to about 7 mol. %. In embodiments where a faster ion exchange time is desired for a given temperature, the concentration of Li₂O may be greater than or equal to about 1 mol. % and less than or equal to about 5 mol. %, or even greater than or equal to about 2 mol. % and less than or equal to about 5 mol. %.

Additions of Na₂O in the glass compositions facilitate ion exchange strengthening the glass compositions. Specifically, the smaller Na⁺ ions in the resultant glass network can be exchanged for larger K⁺ ions in the ion exchange salt bath. If the Na₂O concentration in the glass composition is too low, the resultant depth of layer after ion exchange is too low. However, if the Na₂O concentration in the glass composition in too high, the HT CTE of the glass composition increases. In the embodiments described herein, the Na₂O is present in the glass compositions in an amount from about 3 mol. % to about 16 mol. %.

In the embodiments of the low boron glass compositions described herein, the concentration of Na₂O in the glass compositions is generally less than or equal to about 16 mol. % in order to maintain a relatively low HT CTE. For example, in some embodiments, the concentration of Na₂O in the glass compositions is greater than or equal to about 6 mol. % and less than or equal to about 16 mol. %. In some embodiments, the concentration of Na₂O in the glass compositions may be greater than or equal to about 8 mol. % and less than or equal to about 16 mol. %. In some other embodiments, the concentration of Na₂O in the glass compositions may be greater than or equal to about 10 mol. % and less than or equal to about 16 mol. %. In still other embodiments, the concentration of Na₂O in the glass compositions may be greater than or equal to about 12 mol. % and less than or equal to about 15 mol. %.

In the embodiments of the high boron glass compositions described herein, the concentration of Na₂O in the glass compositions is generally less than or equal to about 12 mol. % in order to maintain a low alkali to alumina ratio and resultant relatively low HT CTE. For example, in some embodiments, the concentration of Na₂O in the glass compositions is greater than or equal to about 3 mol. % and less than or equal to about 12 mol. %. In some embodiments, the concentration of Na₂O in the glass compositions may be greater than or equal to about 8 mol. % and less than or equal to about 12 mol. %.

In some embodiments, the glass compositions described herein may optionally include the alkali oxide K₂O. K₂O is generally added to the glass compositions to improve ion exchange performance. Specifically, K₂O may be added to the glass compositions in order to achieve the desired compressive stress and DOL. In the embodiments described herein, K₂O, when included, is present in the glass compositions in an amount less than or equal to about 3.0 mol. %.

In the embodiments of the low boron glass compositions described herein, the concentration of K₂O in the glass compositions is generally greater than or equal to about 0 mol. %. For example, in some embodiments, the concentration of K₂O in the glass compositions is greater than or equal to about 0 mol. % and less than or equal to about 5 mol. %. In some embodiments, the concentration of K₂O in the glass compositions may be greater than or equal to about 0 mol. % and less than or equal to about 3 mol. %. In some other embodiments, the concentration of K₂O in the glass compositions may be greater than or equal to about 0 mol. % and less than or equal to about 2 mol. %. In some other embodiments, the concentration of K₂O in the glass compositions may be less than or equal to about 1 mol. % or even less than or equal to about 0.5 mol. %. In some embodiments, the low boron glass compositions are substantially free of K₂O.

In the embodiments of the high boron glass compositions described herein, the concentration of K₂O in the glass compositions is generally greater than or equal to about 0 mol. %. For example, in some embodiments, the concentration of K₂O in the glass compositions is greater than or equal to about 0 mol. % and less than or equal to about 3 mol. %. In some embodiments, the concentration of K₂O in the glass compositions may be greater than or equal to about 0 mol. % and less than or equal to about 2 mol. %. In some other embodiments, the concentration of K₂O in the glass compositions may be less than or equal to about 1 mol. % or even less than or equal to about 0.5 mol. %. In some embodiments, the high boron glass compositions are substantially free of K₂O.

Further, in the embodiments of the high boron glass composition described herein, the ratio of R₂O to the concentration of Al₂O₃ is generally less than or equal to about 1.15 or even 1.1, where R₂O is the sum of the concentrations of Na₂O, Li₂O and K₂O. In some of these embodiments, the ratio of R₂O to the concentration of Al₂O₃ is generally greater than or equal to about 0.9. In some embodiments, the ratio R₂O:Al₂O₃ is less than or equal to about 1.1 and greater than or equal to about 0.9. In some embodiments, the ratio R₂O:Al₂O₃ is less than or equal to about 1.1 and greater than or equal to about 1.0. In some other embodiments, the HT CTE is less than or equal to about 1.0 and greater than or equal to 0.9. Maintaining the ratio R₂O:Al₂O₃ at less than about 1.15 or even 1.1 in high boron glass composition generally lowers the HT CTE to less than or equal to about 27×10−6/° C. In particular, in the embodiments of the glass compositions described herein, the sum of the concentration of alkali oxide constituents is balanced against the concentration of Al₂O₃. This balance produces several desirable characteristics in the resulting glass. Specifically, Al₂O₃ utilizes alkali metals, such as the potassium, lithium and sodium, for charge stabilization. If excess alkali is present in the glass composition (i.e., R₂O:Al₂O₃ is greater than or equal to 1.15 or even 1.1), the excess alkali in the glass composition interacts with the boron in the glass composition and converts the boron from its standard trigonal (threefold-coordinated) configuration into a tetrahedral (fourfold-coordinated) configuration. The change in the coordination of the boron from trigonal to tetrahedral increases the HT CTE of the glass. Accordingly, by maintaining the ratio of R₂O to Al₂O₃ at less than or equal to about 1.15 or even less than or equal to about 1.1 prevents the boron in the glass from assuming a tetrahedral coordination in the glass and thereby promotes a relatively low HT CTE.

In some embodiments described herein, the glass compositions may include B₂O₃. Like SiO₂ and Al₂O₃, B₂O₃ contributes to the formation of the glass network. Conventionally, B₂O₃ is added to a glass composition in order to decrease the viscosity of the glass composition. In general, B₂O₃ acts as a flux which may be utilized to form glass compositions with low softening points. However, the presence of B₂O₃ significantly increases the HT CTE when excess alkali oxides are present which are not associated with Al₂O₃. However, if the concentration of alkali oxides is balanced against the concentration of Al₂O₃, higher concentrations of boron oxide do not significantly impact the HT CTE of the glass composition. Accordingly, the glass compositions described herein may contain a low concentration of B₂O₃ (i.e., the “low boron glass compositions”) or a high concentration of B₂O₃ (i.e., the “high boron glass compositions”) in order to achieve a glass composition having the desired properties.

In the low boron glass compositions, B₂O₃ is generally present in the glass compositions in an amount less than or equal to about 1 mol. % in order to minimize the increase in HT CTE by limiting the interaction between B₂O₃ and excess alkali oxides. For example, in some embodiments, B₂O₃ is present in the glass compositions in a concentration greater than or equal to about 0 mol. % and less than or equal to about 1 mol. %. In other embodiments described herein, B₂O₃ is present in the glass compositions in a concentration of less than about 0.5 mol. %. For example, in some embodiments, the concentration of B₂O₃ in the glass composition is greater than or equal to about 0 mol. % and less than or equal to about 0.5 mol. %, or even less than or equal to about 0.4 mol. %.

In the high boron glass compositions, B₂O₃ is generally present in the glass compositions in an amount greater than or equal to about 4.5 mol. %. In these embodiments, the impact of B₂O₃ on the HT CTE is mitigated by controlling the ratio R₂O:Al₂O₃, as described above. For example, in some embodiments, B₂O₃ is present in the glass compositions in a concentration greater than or equal to about 4.5 mol. % or even 5 mol. % and less than or equal to about 12 mol. %. In other embodiments described herein, the concentration of B₂O₃ in the glass composition is greater than or equal to about 7 mol. % and less than or equal to about 12 mol. %, or even greater than or equal to about 9 mol. % and less than or equal to about 12 mol. %.

The glass compositions described herein may further include one or more divalent oxides MO, where M is an alkaline earth metal (such as Mg or Ca) and/or Zn. The divalent oxides improve the melting behavior of the glass compositions. Additions of MgO and ZnO also improve the ion exchange performance of the glass composition. In particular, it has been found that additions of MgO and ZnO generally increase the compressive stress and DOL for a given ion exchange condition (time and temperature) without increasing the softening point of the glass composition. Additions of CaO to the glass composition generally assist in maintaining a sufficient DOL of the compressive stress following ion exchange strengthening.

In the low boron glass compositions described herein, the glass compositions include at least one of MgO and ZnO and the total concentration of divalent oxide is greater than or equal to about 0.8 mol. % or even 1 mol. % and less than or equal to about 10 mol. %. Moreover, in the low boron glass compositions described herein, the sum of the concentration of Al₂O₃ (mol. %) and the concentration of the divalent oxide (mol. %) is generally greater than or equal to about 10 mol. % which generally improves the ion exchange performance of the glass.

In the low boron glass compositions described herein, MgO may be present in a concentration from about 0 mol. % to about 7 mol. %. For example, in some embodiments, the concentration of MgO may be greater than or equal to about 3 mol. % and less than or equal to about 5 mol. %. In some other embodiments, the concentration of MgO may be greater than or equal to about 2 mol. % and less than or equal to about 4 mol. %.

Further, in the low boron glasses described herein, CaO may be present in a concentration greater than or equal to about 0 mol. % and less than or equal to about 1 mol. %. For example, in some embodiments, the concentration of CaO may be greater than or equal to about 0 mol. % and less than or equal to about 0.5 mol. %.

In the low boron glass compositions described herein, ZnO may be present in a concentration from about 0 mol. % to about 6 mol. %. For example, in some embodiments, the concentration of ZnO may be greater than or equal to about 2 mol. % and less than or equal to about 4 mol. %.

The high boron glass compositions described herein may also include one or more divalent oxides MO. For example, in some embodiments of the high boron glass compositions which include MgO, the MgO may be present in a concentration greater than or equal to about 0 mol. % and less than or equal to about 5 mol. %. In some embodiments of the high boron glass compositions which include ZnO, the ZnO may be present in a concentration greater than or equal to about 0 mol. % and less than or equal to about 5 mol. %. In some embodiments of the high boron glass compositions which include CaO, the CaO may be present in a concentration greater than or equal to about 0 mol. % and less than or equal to about 2 mol. %.

The glass compositions described herein (i.e. both low boron glass compositions and high boron glass compositions) may also include P₂O₅. Additions of P₂O₅ increase the speed of ion exchange at a given temperature such that an equivalent depth of layer may be reached in a shorter time period. In some embodiments of the glass compositions described herein, P₂O₅ may be present in the glass compositions in a concentration greater than or equal to about 0 mol. % and less than or equal to about 3 mol. %. In some embodiments, the concentration of P₂O₅ may be greater than or equal to about 0 mol. % and less than or equal to about 2 mol. %. In some other embodiments, the concentration of P₂O₅ may be greater than or equal to about 0.5 mol. % and less than or equal to about 1.0 mol. %.

The glass compositions described herein may optionally include one or more fining agents. The fining agents may include, for example, SnO₂, Sb₂O₃, As₂O₃, NaCl, (Al)OH₃, and CeO₂, and combinations thereof. The fining agents may be present in the glass compositions in an amount greater than or equal to about 0 mol. % and less than or equal to about 1.0 mol. %. In exemplary embodiments, the fining agent is SnO₂. The SnO₂ may be present in the glass composition in a concentration greater than or equal to about 0 mol. % and less than or equal to about 1.0 mol. %. In some of these embodiments, SnO₂ may be present in the glass composition in a concentration which is greater than or equal to about 0 mol. % and less than or equal to about 0.5 mol. % or even less than or equal to about 0.3 mol. %.

In some embodiments of the low boron glass compositions described herein, the glass compositions may optionally comprise zirconia (ZrO₂). Additions of zirconia improve the ion exchange performance of the glass composition by increasing the depth of layer which may be achieved. However, if the amount of zirconia exceeds about 3 mol. %, the liquidus viscosity of the glass composition decreases, making the glass composition difficult to form. Accordingly, in the embodiments of the glass compositions which contain zirconia, the glass compositions may include greater than or equal to about 0.5 mol. % and less than or equal to about 2 mol. % ZrO₂. In some of these embodiments, the low boron glass compositions may include greater than or equal to about 1.0 mol. % and less than or equal to about 1.5 mol. % ZrO₂. In some of these embodiments, the concentration of ZrO₂ in the low boron glass compositions is about 1 mol. %. However, it should be understood that, in some embodiments of the low boron glass compositions described herein, the glass compositions are substantially free of zirconia (ZrO₂).

Certain applications of the glass compositions described herein may require that the glass be opaque and have a certain color, such as black. Accordingly, in the embodiments of the high boron glass compositions described herein, the glass compositions may include one or more constituents which act as a colorant. For example, some of the high boron glass compositions may include Fe₂O₃ and TiO₂ which, in combination, may be used to impart a black color and opacity to glass formed from the glass compositions. In these embodiments, Fe₂O₃ and TiO₂ may be present in the glass composition such that a ratio of Fe₂O₃ (mol. %) to TiO₂ (mol. %) is in a range from greater than or equal to about 0.52 to less than or equal to about 1.22. In some embodiments, the ratio of Fe₂O₃ (mol. %) to TiO₂ (mol. %) may be in a range from greater than or equal to about 0.60 to less than or equal to about 1.00. As the ratio of Fe₂O₃ to TiO₂ decreases, the resultant glass article becomes less black as determined from the CIE F2 Illuminant and the L, a*, b* scale. In one particular embodiment, the total concentration of Fe₂O₃ and TiO₂ (i.e., Fe₂O₃ (mol. %)+TiO₂ (mol. %)) is approximately 1.75 mol. %. However, it should be understood that other total concentrations of Fe₂O₃ and TiO₂ may also be utilized including total concentrations greater than 1.75 mol. % and total concentrations less than 1.75 mol. %.

In some embodiments described herein, a black color is achieved in the resultant glass formed from a glass composition containing color modifying constituents by thermally treating the glass article. For example, in one embodiment, the glass may be initially annealed at a temperature in the range from about 560° C. to about 575° C. for a time period of less than or equal to approximately 1 hour and subsequently cooled. Thereafter, the glass may be heat treated at a temperature from about 600° C. to about 650° C. for less than or equal to approximately 10 hours, after which the glass is opaque and has a black color. While not wishing to be bound by theory, it is believed that the black color is a result of precipitates formed from a combination of Fe₂O₃ and TiO₂ (such as pseudo-brookite or the like) which precipitate in the glass during thermal treatment.

In some embodiments, when the high boron glass composition includes color modifying constituents such as those described above, the black color following thermal treatment occurs when the alkali to alumina ratio (R₂O:Al₂O₃) is less than or equal to about 1.15 or even less than or equal to about 1.12. In some embodiments, the black color may be obtained when the alkali to alumina ratio (R₂O:Al₂O₃) is greater than or equal to about 0.98 or even 1.02. For example, in some embodiments, the black color may be obtained when the alkali to alumina ratio is less than or equal to about 1.15 and greater than about 0.98. In some embodiments, the black color is obtained when the alkali to alumina ratio is less than or equal to about 1.12 and greater than or equal to about 1.02. In some other embodiments, the black color is obtained when the alkali to alumina ratio less than or equal to about 1.1 and greater than or equal to about 1.04.

In the embodiments of the high boron glass compositions which include color modifying constituents, the resultant glass may be opaque. The degree of opacity may be determined by the light absorption of the glass as measured by spectral photometry. In the embodiments described herein, the degree of opacity was measured by an X-Rite CI7 Spectro-Photometer. In the exemplary embodiments of the high boron glass with color modifying agents described herein, the resultant opaque glass may have an opacity (i.e., a light absorption) greater than or equal to about 80% over a range of wavelengths from about 350 nm to about 750 nm incident on the glass article. This means that less than about 20% of the light incident on the glass article is actually transmitted through the glass article. In some embodiments, the opacity may be greater than or equal to about 80% and less than or equal to 100% over a range of wavelengths from about 350 nm to about 750 nm. In some other embodiments, the opacity may be about 100% over a range of wavelengths from about 350 nm to about 750 nm.

As noted herein, color modifying agents added to the high boron glass compositions may result in a glass which is black in color. The degree of color may be quantified according using the CIE F2 Illuminant and the L, a*, b* scale. For example, in some embodiments, the glass article has L, a*, b*, color coordinates in which L from about 0 to about 5.0; a* from about −2.0 to about 2.0; and b* from about 0 to about −5.0. Glass with color coordinates within these ranges generally has a deep black color.

In addition, it has been found that the high boron glass compositions which contain color modifying constituents as described herein are readily amenable to strengthening by ion exchange. The depth of layer of these glass compositions may be greater than or equal to about 25 μm. In some embodiments, the DOL may be greater than or equal to about 35 μm or even greater than or equal to about 45 μm. It is believed that the compressive stress imparted to these glass compositions by ion exchange may be at least equal to or even greater than the same glass composition without color modifying agents. However, measurement of the compressive stress using conventional techniques (such as stress birefringence) is complicated due to the optical properties of the glass (opacity and black color). Accordingly, the characteristic strength of the glass article may be used as an estimate of the compressive stress imparted to the glass. Specifically, ring-on-ring testing of a plurality of un-abraded glass plate samples formed from the glass composition may be tested using the methodology described in ASTM Standard C1499 entitled “Standard Test Method for Monotonic Equibiaxial Flexural Strength of Advanced Ceramics at Ambient Temperature.” From this data a Weibull distribution of the strength at failure may be constructed and the characteristic strength and Weibull modulus may be determined. In the exemplary embodiments described herein, a glass plate formed from the high boron glass composition containing color modifying agents with a thickness of approximately 0.8 mm generally has a characteristic strength greater than or equal to about 1500 MPa or even about 1600 MPa following an anneal treatment at about 570° C. for approximately 2 hours, a heat treatment at about 640° C. for approximately 4 hours and an ion exchange treatment at about 440° C. for approximately 15 hours in a salt bath of 100% KNO₃. In some embodiments, the characteristic strength may be greater than or equal to about 1700 MPa or even about 1800 MPa after the same treatments. In some other embodiments the characteristic strength may be greater than or equal to about 1900 MPa after the same treatments. In the embodiments described herein, a glass plate formed from the high boron glass composition containing color modifying agents with a thickness of approximately 0.8 mm generally has a Weibull modulus greater than or equal to about 8 or even about 9 following an anneal treatment at about 570° C. for approximately 2 hours, a heat treatment at about 640° C. for approximately 4 hours and an ion exchange treatment at about 440° C. for approximately 15 hours in a salt bath of 100% KNO₃. In embodiments, the Weibull modulus may be greater than or equal to about 10 or even about 11 after the same treatments. The Weibull modulus is the slope of the Weibull plot and is generally indicative of sensitivity of the material to failure due to flaws. In embodiments the characteristic strength may be greater than or equal to about 13 after the same treatments. The characteristic strength is indicative of the strength at 63.2% the failure probability as determined by the Weibull distribution.

In addition to the relatively low softening points, HT CTEs, and ion exchange properties described above, the glass compositions described herein also exhibit properties which make the glass compositions suitable for use in fusion forming processes, such as the fusion down draw process. Specifically, the glass compositions described herein have liquid temperatures of less than about 1000° C. and liquidus viscosities of greater than about 150 kP. Moreover, the glass compositions also have zircon breakdown viscosities of less than about 35 kP such that the glass compositions are compatible with fusion forming utilizing zirconia isopipes. Further, the glass compositions described herein also exhibit a viscosity of less than about 200 P at melting temperatures in the range from about 1500° C. to about 1650° C. and viscosities of about 35 kP at forming temperatures from about 1050° C. to about 1150° C.

Based on the foregoing, it should be understood that various embodiments of glass compositions with relatively low softening points and relatively low high temperature coefficients of thermal expansion are disclosed herein. In a first exemplary embodiment, a glass composition includes SiO₂, Al₂O₃, Li₂O and Na₂O. The glass composition may generally have a softening point less than or equal to about 810° C. and a high temperature CTE less than or equal to about 27×10⁻⁶/° C. The glass composition may have a compressive stress greater than or equal to about 650 MPa and a depth of layer greater than or equal to about 25 μm after ion exchange in a salt bath comprising KNO₃ at about 410° C. for less than or equal to approximately 15 hours.

In a second exemplary embodiment, a glass composition includes from about from about 65 mol. % to about 71 mol. % SiO₂; from about 7 mol. % to about 12 mol. % Al₂O₃; from about 1 mol. % to about 9 mol. % Li₂O; from about 6 mol. % to about 16 mol. % Na₂O; from about 0 mol. % to about 5 mol. % K₂O; from about 0.8 to about 10 mol. % of a divalent oxide, wherein the divalent oxide comprises at least one of MgO and ZnO; and less than about 0.5 mol. % B₂O₃, wherein a sum of the concentration of Al₂O₃ (mol. %), and a concentration of the divalent oxide (mol. %) is greater than about 10 mol %. In this second exemplary embodiment, the glass composition may optionally include from about 0.5 mol. % to about 2 mol. % ZrO₂. Alternatively, this second exemplary composition may be substantially free of ZrO₂. When the second exemplary compositions is substantially free of ZrO₂, the glass composition may comprise from about 65.8 mol. % to about 71 mol. % SiO₂. The glass composition may generally have a softening point less than or equal to about 810° C. and a high temperature CTE less than or equal to about 27×10⁻⁶/° C. The glass composition may have a compressive stress greater than or equal to about 650 MPa and a depth of layer greater than or equal to about 25 μm after ion exchange in a salt bath comprising KNO₃ at about 410° C. for less than or equal to approximately 15 hours.

In a third exemplary embodiment, a glass composition includes from about from about 55 mol. % to about 68 mol. % SiO₂; from about 9 mol. % to about 15 mol. % Al₂O₃; from about 4.5 mol % to about 12 mol. % B₂O₃; from about 1 mol. % to about 7 mol. % Li₂O; from about 3 mol. % to about 12 mol. % Na₂O; and from about 0 mol. % to about 3 mol % K₂O. In this embodiment, R₂O is the sum of the concentration of Li₂O, the concentration of Na₂O, and the concentration of K₂O, and the ratio of R₂O to the concentration of Al₂O₃ is less than or equal to about 1.1. The glass composition may generally have a softening point less than or equal to about 810° C. and a high temperature CTE less than or equal to about 27×10⁻⁶/° C. The glass composition may have a compressive stress greater than or equal to about 650 MPa and a depth of layer greater than or equal to about 25 μm after ion exchange in a salt bath comprising KNO₃ at about 410° C. for less than or equal to approximately 15 hours.

In a fourth exemplary embodiment, a glass composition includes from about 65.8 mol. % to about 71 mol. % SiO₂; from about 7 mol. % to about 12 mol. % Al₂O₃; from about 1 mol. % to about 9 mol. % Li₂O; from about 6 mol. % to about 16 mol. % Na₂O; from about 1 mol. % to about 10 mol. % of a divalent oxide, wherein the divalent oxide is at least one of MgO and ZnO; and less than about 0.5 mol. % B₂O₃. The sum of the concentration of Al₂O₃ (mol. %), and a concentration of the divalent oxide (mol. %) is greater than about 10 mol %. The glass composition may generally have a softening point less than or equal to about 810° C. and a high temperature CTE less than or equal to about 27×10⁻⁶/° C. In this embodiment, the glass composition may be substantially free of ZrO₂.

In a fifth exemplary embodiment, a glass composition includes from about 65.8 mol. % to about 71 mol. % SiO₂; from about 7 mol. % to about 12 mol. % Al₂O₃; from about 1 mol. % to about 9 mol. % Li₂O; from about 6 mol. % to about 16 mol. % Na₂O; and from about 1 mol. % to about 10 mol. % of a divalent oxide, wherein the divalent oxide is at least one of MgO and ZnO. The glass composition is substantially free from ZrO₂ and B₂O₃. The sum of the concentration of Al₂O₃ (mol. %), and the concentration of the divalent oxide (mol. %) is greater than 10 mol %. The glass composition may generally have a softening point less than or equal to about 810° C. and a high temperature CTE less than or equal to about 27×10⁻⁶/° C.

In a sixth exemplary embodiment, a glass composition includes from about 67 mol. % to about 71 mol. % SiO₂; from about 7 mol. % to about 12 mol. % Al₂O₃; from about 1 mol. % to about 9 mol. % Li₂O; from about 6 mol. % to about 16 mol. % Na₂O; and from about 1 mol. % to about 10 mol. % of a divalent oxide, wherein the divalent oxide is at least one of MgO and ZnO. The glass composition may also include from about 0 mol. % to about 7 mol. % MgO; from about 0 mol. % to about 1 mol. % CaO; and from about 0 mol. % to about 6 mol. % ZnO. The glass composition is substantially free from B₂O₃. The sum of the concentration of Al₂O₃ (mol. %), and the concentration of the divalent oxide (mol. %) is greater than about 10 mol %. The glass composition may generally have a softening point less than or equal to about 810° C. and a high temperature CTE less than or equal to about 27×10⁻⁶/° C.

In a seventh exemplary embodiment, a glass composition includes from about from about 55 mol. % to about 68 mol. % SiO₂; from about 9 mol. % to about 15 mol. % Al₂O₃; from about 4.5 mol % to about 12 mol. % B₂O₃; from about 1 mol. % to about 7 mol. % Li₂O; from about 3 mol. % to about 12 mol. % Na₂O; and from about 0 mol. % to about 3 mol % K₂O. In this embodiment, R₂O is the sum of the concentration of Li₂O, the concentration of Na₂O, and the concentration of K₂O, and the ratio of R₂O to the concentration of Al₂O₃ is less than or equal to about 1.1. The glass composition may generally have a softening point less than or equal to about 810° C. and a high temperature CTE less than or equal to about 27×10⁻⁶/° C.

In an eighth exemplary embodiment, a glass composition includes from about 55 mol. % to about 68 mol. % SiO₂; from about 9 mol. % to about 15 mol. % Al₂O₃; from about 4.5 mol % to about 12 mol. % B₂O₃; from about 1 mol. % to about 7 mol. % Li₂O; from about 3 mol. % to about 12 mol. % Na₂O; and from about 0 mol. % to about 3 mol % K₂O. In this embodiment, R₂O is the sum of the concentration of Li₂O, the concentration of Na₂O, and the concentration of K₂O, and the ratio of R₂O to the concentration of Al₂O₃ is less than or equal to about 1.1. The glass composition may generally have a softening point less than or equal to about 810° C. and a high temperature CTE less than or equal to about 27×10⁻⁶/° C.

In a ninth exemplary embodiment, a glass composition includes from about 55 mol. % to about 68 mol. % SiO₂; from about 9 mol. % to about 15 mol. % Al₂O₃; from about 4.5 mol % to about 12 mol. % B₂O₃; from about 1 mol. % to about 7 mol. % Li₂O; from about 3 mol. % to about 12 mol. % Na₂O; and from about 0 mol. % to about 3 mol % K₂O. The glass composition may further include from about 0 mol. % to about 5 mol. % MgO; from about 0 mol. % to about 5 mol. % ZnO; and from about 0 mol. % to about 2 mol. % CaO. In this embodiment, R₂O is the sum of the concentration of Li₂O, the concentration of Na₂O, and the concentration of K₂O and the ratio of R₂O to the concentration of Al₂O₃ is less than or equal to about 1.1. The glass composition may generally have a softening point less than or equal to about 810° C. and a high temperature CTE less than or equal to about 27×10⁻⁶/° C.

In a tenth exemplary embodiment, a glass composition includes from about 65 mol. % to about 71 mol. % SiO₂; from about 7 mol. % to about 12 mol. % Al₂O₃; from about 1 mol. % to about 9 mol. % Li₂O; from about 6 mol. % to about 16 mol. % Na₂O; from about 0 mol. % to about 5 mol. % K₂O; from about 0.8 to about 10 mol. % of a divalent oxide, wherein the divalent oxide comprises at least one of MgO and ZnO; from about 0.5 mol. % to about 2 mol. % ZrO₂; and less than about 0.5 mol. % B₂O₃. In this exemplary embodiment, the sum of the concentration of Al₂O₃ (mol. %), and the concentration of the divalent oxide (mol. %) is greater than about 10 mol %. The glass composition has a softening point less than or equal to about 810° C. The glass composition has a high temperature CTE less than or equal to about 27×10⁻⁶/° C.

EXAMPLES

The embodiments of the glass compositions described herein will be further clarified by the following examples.

A plurality of exemplary glass compositions were prepared according to the batch compositions listed in Tables 1-6 below. Batches of the oxide constituent components were mixed, melted and formed into glass. The properties of the glass composition (i.e., softening point, HT CTE, etc.) were measured and the results are reported in Tables 1-6. Comparative Examples (i.e., non-inventive Examples) are prefaced with a “C” in the example identification.

Referring now to Table 1, seven exemplary low boron glass compositions were prepared to investigate the effect of substituting Li₂O for Na₂O on softening point and HT CTE. FIG. 2 graphically depicts the softening point (y-axis) as a function of the concentration of Li₂O (x-axis) as Li₂O is substituted for Na₂O. FIG. 3 graphically depicts the HT CTE (y-axis) as a function of the concentration of Li₂O (x-axis) as Li₂O is substituted for Na₂O.

As indicated by the data in Table 1 and FIGS. 2 and 3, the partial substitution of Li₂O for Na₂O resulted in a progressive decrease in the softening point of the glass composition. In particular, substitution of 9 mol. % Li₂O for Na₂O decreased the softening point by up to 70° C. The substitution of Li₂O for Na₂O also caused a slight increase in the HT CTE (from 24×10⁻⁶/° C. to 25.5×10⁻⁶/° C. for a 1-3 mol. % substitution). However, even after the substitution, the HT CTE values of these compositions were less than 27×10⁻⁶/° C. Moreover, as graphically depicted in FIG. 3, the data from Table 1 indicates that the increase in HT CTE levels off for increasing concentrations of Li₂O.

TABLE 1 Substitution of Li₂O for Na₂O (Mol % ) 1 2 3 4 5 6 7 SiO₂ 69.4 69.4 69.4 69.4 69.4 69.4 69.4 Al₂O₃ 11.2 11.2 11.2 11.2 11.2 11.2 11.2 B₂O₃ 0.3 0.3 0.3 0.3 0.3 0.3 0.3 Na₂O 14.2 13.2 11.2 10.2 9.2 7.7 6.2 K₂O 3.0 3.0 3.0 3.0 3.0 3.0 3.0 Li₂O 1.0 2.0 4.0 5.0 6.0 7.5 9.0 CaO 0.3 0.3 0.3 0.3 0.3 0.3 0.3 MgO 0.3 0.3 0.3 0.3 0.3 0.3 0.3 ZnO 0.3 0.3 0.3 0.3 0.3 0.3 0.3 SnO₂ 0.2 0.2 0.2 0.2 0.2 0.2 0.2 Softening pt, 792 778 751 751 748 742 732 ° C. HT CTE, 24.4 25.3 25.7 25.1 25 25.3 25.3 ppm/° C.

Referring now to Table 2, eight exemplary low boron glass compositions were prepared to investigate the effect of substituting Li₂O for Na₂O and Na₂O+K₂O on softening point and HT CTE. FIG. 4 graphically depicts the softening point (y-axis) as a function of the concentration of Li₂O (x-axis) as Li₂O is substituted for Na₂O+K₂O. FIG. 5 graphically depicts the HT CTE (y-axis) as a function of the concentration of Li₂O (x-axis) as Li₂O is substituted for Na₂O+K₂O.

As indicated by the data in Table 2 and FIGS. 4 and 5, the partial substitution of Li₂O for Na₂O or Na₂O+K₂O resulted in a progressive decrease in the softening point of the glass composition. In particular, substitution of 5 mol. % Li₂O for Na₂O+K₂O decreased the softening point by up to about 90° C. The substitution of Li₂O for Na₂O+K₂O also caused a slight increase in the HT CTE. However, even after the substitution, the HT CTE values of these compositions were less than 27×10⁻⁶/° C. No leveling of the HT CTE was observed as with the glass compositions of Table 2.

TABLE 2 Substitution of Li₂O for Na₂O and K₂O (Mol %) C1 C2 8 9 10 11 12 13 SiO₂ 69.8 69.8 69.8 69.8 69.8 69.8 69.8 69.8 Al₂O₃ 9.0 9.0 9.0 9.0 9.0 9.0 9.0 9.0 Li₂O 1.0 1.5 3.0 3.5 4.9 4.9 4.9 5.0 Na₂O 12.7 12.2 10.7 10.2 10.0 10.0 10.0 8.7 K₂O 1.7 1.7 1.7 1.7 0.7 0.7 0.7 1.7 MgO 5.1 5.1 5.1 5.1 5.1 5.1 5.1 5.1 CaO 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 SnO₂ 0.2 0.2 0.2 0.2 0.3 0.3 0.2 0.2 total 15.4 15.4 15.4 15.4 15.6 15.6 15.6 15.4 R₂O softening 833 822 799 791 770 779 781 777 pt HT CTE 22.3 21.8 22.5 23.3 24.3 23.8 23.7 23.8

Table 3 contains the post-ion exchange properties (compressive stress and DOL) for several of the exemplary glass compositions listed in Table 2 for a variety of different ion exchange processing conditions. The ion exchange conditions included single ion exchange treatments in 100% KNO₃ at a variety of temperature and immersion times, as well as two step treatments involving a mixed bath (e.g., 90% KNO₃-10% NaNO₃), followed by immersion in a second bath of 100% KNO₃. The compressive stress and DOL values shown in Table 3 are measured values obtained with a FSM instrument, with the compressive stress value being based on the measured stress optical coefficient (SOC). FIG. 6 graphically depicts the compressive stress (y-axis) and DOL (x-axis) for different Li₂O concentrations (i.e., 1.0 mol. %, 3.5 mol. %, and 4.9 and 5.0 mol. % (collectively)). As shown in FIG. 6, a 40 μm DOL can be achieved for the 1% and 3.5% Li₂O concentrations, but not for greater concentrations, thereby indicating that, at least for Li-for-Na substitution, there may be an effective upper limit to the amount of Li₂O that can be introduced into the glass while still achieving the desired DOL.

TABLE 3 Ion Exchange Properties of Select Compositions 9 11 12 13 Li₂O (mole %) 3.5 4.9 4.9 5.0 IX Schedule, CS (MPa), DOL (um) 410-8 h 100KNO₃ 836, 19 820, 19 805, 21 410-10 h 100KNO₃ 803, 24 816, 18 806, 26 410-12 h 100KNO₃ 783, 25 803, 23 410-15 h 100KNO₃ 801, 28 775, 26 420-6 h 100KNO₃ 782, 24 803, 18 420-8 h 100KNO₃ 772, 27 773, 21 762, 22 420-15 h 100KNO₃ 765, 32 430-15 h 100KNO₃ 768, 36 707, 37 440-15 100KNO₃ 715, 42 390-15 h 80KNO₃/20NaNO₃ + 853, 19 886, 14 904, 16 430-1 h 100 KNO₃ 390-15 h 90KNO₃/10NaNO₃ + 889, 14 897, 16 710, 19 430-1 h 100 KNO₃ 410-15 h 80KNO₃/20NaNO₃ + 759, 24 897, 18 882, 18 710, 22 430-1 h 100 KNO₃ 410-15 h 90KNO₃/10NaNO₃ + 759, 27 687, 25 430-1 h 100 KNO₃ 649, 24

Referring now to Table 4, Table 4 contains the compositional data and corresponding softening points, HT CTEs, and ion exchange characteristics of several inventive and comparative examples of low boron glass compositions.

TABLE 4 Low Boron Glass Compositions (Mol %) C3 C4 14 15 16 17 18 19 20 21 22 C5 SiO₂ 65.8 65.8 67.8 67.8 65.8 69.4 69.4 69.4 69.6 68.2 67.3 65.9 Al₂O₃ 10.0 10.0 10.0 10.0 8.0 11.2 10.7 10.7 7.5 7.3 9.0 9.0 Li₂O 4.0 3.0 3.0 3.0 3.0 3.0 1.0 3.0 2.1 2.1 3.5 3.5 Na₂O 12.0 13.0 15.0 15.0 13.0 15.4 15.2 13.2 14.1 13.4 14.6 14.6 K₂O 0 0 0 0 0 0 2.0 2.0 0 2.0 0.5 0.5 MgO 4.0 4.0 2.0 0 5.0 0.5 0.5 0.5 6.6 6.4 5.1 6.5 CaO 0 0 0 0 0 0 0 0 0 0.5 0 0 ZnO 4.0 4.0 2.0 4.0 5.0 0.5 0.5 0.5 0 0 0 0 P₂O₅ 0 0 0 0 0 0 0.8 0.8 0 0 0 0 SnO₂ 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 MgO + 8.0 8.0 4.0 4.0 10.0 1.0 1.0 1.0 6.6 6.4 5.1 6.5 ZnO Al₂O₃ + 18.0 18.0 14.0 14.0 18.0 12.2 11.7 11.7 14.1 13.7 14.1 15.5 MgO + ZnO Softening 778 785 757 764 752 775 802 785 781 747 750 742 pt, ° C. HT CTE, 24.0 21.8 24.2 24.3 24.0 22.7 23.2 23.9 25.6 26.6 25.6 ppm/° C. IX, 430- 15 h (100 KNO₃) CS (MPa) 868 950 637 872 DOL (um) 31 32 43 28 IX, 430- 15 h (100 KNO₃) CS (MPa) 796 893 587 DOL (um) 36 37 50 IX, 410- 15 h (90 KNO₃/10 NaNO₃) CS (MPa) 690 720 662 760 DOL (um) 62 48 38 34

Referring now to Table 5, a series of low boron glass composition containing zirconia were also formed. The softening points of these glass compositions were less than 800° C. and the HT CTEs were less than 27×10⁻⁶/° C. Following ion exchange in a salt bath of 100% KNO₃ at 440° C. for 15 hours the glass compositions had a compressive stress greater than 700 MPa and a depth of layer of 40 μm.

TABLE 5 Glass Compositions with Zirconia Additions (Mol %) 23 24 25 26 27 28 SiO₂ 65.9 65.4 65.9 65.9 65.9 64.9 Al₂O₃ 9 9 9 8 8.5 8.5 Li₂O 3.5 3.5 3.5 3.5 3.5 3.5 Na₂O 14.6 14.6 16.1 16.1 16.1 16.1 K₂O 0.5 0.5 0.5 0.5 0.5 0.5 MgO 5.5 3 3.5 3.5 3 3 ZnO 0 3 0 0.5 1 2 P₂O₅ 0 0 0 1 0 0 ZrO₂ 1 1 1.5 1 1.5 1.5 SnO₂ 0.2 0.2 0.2 0.2 0.2 0.2 Soft pt 771 758 752 751 744 744 (° C.) HT CTE 24.6 24.4 25.0 26.1 25.4 25.3 (ppm) IX (100% 665, 43 721, 42 505, 49 440, 63 489, 49 523, 46 KNO₃ @ 440° C. for 15 hrs) CS, DOL

Referring to Table 6, two inventive compositions (29 and 30) were subjected to additional characterization and a determination of whether the glass compositions would be suitable for use with fusion forming processes. Table 6 contains the composition and properties of the inventive compositions as well as two comparative examples of ion exchangeable glass compositions suitable for use with a fusion forming process. The inventive low boron glass compositions had softening points that are approximately 40° C. lower than the softening points of the comparative glass compositions. Further, the HT CTE values of the low boron glass compositions are comparable to or less than the HT CTE values of the comparative glass compositions. The inventive low boron glass compositions have similar ion exchange properties as well as high temperature viscosities, liquidus temperatures, liquidus viscosities and zircon breakdown temperatures as the comparative glass compositions indicating that the inventive low boron glass compositions would be suitable for use with fusion forming processes.

TABLE 6 Forming Characteristics of Inventive and Comparative Glass Compositions Mol % C6 C7 29 30 SiO₂ 66.0 69.2 69.8 66.8 Al₂O₃ 10.3 8.5 9.2 10.0 B₂O₃ 0.6 0 0 0 Li₂O 0 0 3.5 3.0 Na₂O 14.2 13.9 11.9 14.0 K₂O 2.4 1.2 0.5 0 MgO 5.8 6.5 5.1 3.0 CaO 0.6 0.5 0 0 ZnO 0 0 0 3.0 SnO₂ 0.2 0.2 0.2 0 soft pt 837 844 799 781 HT CTE 23.0 21.0 22.6 24.0 DOL 40 40 40 44 CS 740 740 680 785 HTV: 200 P 1588 1640 1609 1567 HTV: 35 kP 1131 1145 1117 1084 delta 457 495 492 483 Liquidus temp 900 950 965 970° Liquidus viscosity >1e6 >1e6 500 kP 275 kP viscosity (RT) 2.461 2.444 2.438 2.497

Referring now to Table 7, Table 7 contains the compositional data and corresponding softening points and HT CTEs for inventive and comparative high boron glass compositions. As shown in Table 7, the inventive glass compositions generally have greater than about 5 mol. % and, in the examples shown, up to about 10 mol % B₂O₃. However, for the inventive high boron glass compositions shown in Table 7, the ratio R₂O:Al₂O₃ is generally greater than or equal to about 0.9 and less than or equal to about 1.15.

Comparative Example C8 does not contain Li₂O and, as a result, the softening point for this glass is greater than the inventive high boron glass compositions illustrating the need to have Li₂O in the glass compositions.

TABLE 7 High Boron Glass Compositions (Mol %) C8 31 32 33 34 35 36 37 SiO₂ 64.2 60 58.6 65.7 65.7 65.7 65.2 63.7 Al₂O₃ 12.0 14.4 13.8 12.3 12.3 11.3 12.3 12.3 B₂O₃ 9.0 5.2 5.1 9.1 9.1 9.1 9.1 9.1 Li₂O 0 6.1 4.9 5.0 7.0 5.0 5.0 5.0 Na₂O 11.3 7.6 8.2 6.6 4.6 6.6 6.6 6.6 K₂O 0.5 0.5 0.5 1.3 1.3 1.3 1.3 1.3 MgO 1.5 2.7 2.7 0 0 1 0 0 CaO 0 0 0 0 0 0 0 0 ZnO 1.5 3.5 3.9 0 0 0 0 0 P₂O₅ 0 0 2.3 0 0 0 0.5 2 SnO₂ 0.3 0.3 0.3 0.0 0.0 0.1 0.1 0.1 R₂O/ 0.98 0.99 0.99 1.05 1.05 1.14 1.05 1.05 Al₂O₃ softening 845 770 781 785 786 745 795 789 pt. HT CTE 24.6 26.7 25.4 26.7 31.4 25.1 26.0

Referring now to Table 8, five glass compositions were formulated containing the color modifying constituents TiO₂ and Fe₂O₃ to produce an opaque, black glass. Inventive compositions 38-41 were based on composition 31 from Table 7 and inventive composition 42 was based on composition 34 of Table 7. The alkali to alumina ratio of each glass composition was less than 1.15 and greater than 1.0. Plate samples having a thickness of 0.8 mm were produced and measured. Each of compositions 39-42 exhibited a softening point of less than about 810° C. with composition 38 having a softening point of approximately 814° C. which is about 810° C. given the measurement error (+/−5° C.) of the instrument. Compositions 38 and 41-42 also exhibited an HT CTE of less than or equal to about 27×10⁻⁶/° C. (the HT CTE for compositions 39 and 40 was not measured). Each sample was annealed at 570° C. for 2 hours, cooled to room temperature and heat treated at 640° C. for 4 hours to produce the black color. The opacity of the samples was qualitatively assessed by shining a light source on the surface of the plate and qualitatively determining if the light was transmitted through the other side of the plate. As indicated in Table 8, all samples appeared opaque based on this qualitative assessment. In addition the L, a*, b*, coordinates were determined for compositions 38, 41, and 42 using an X-Rite CI7 Spectro-Photometer. Samples of compositions 41 and 42 were also ion exchanged in a 100% KNO₃ salt bath at 430° C. for 15 hours. It was determined that the resultant depth of layer of potassium diffusion in each sample was greater than 30 μm.

TABLE 8 High Boron Glass Compositions with Color Modifying Constituents (Mol %) 38 39 40 41 42 SiO₂ 61.0 62.0 62.0 63.5 65.7 Al₂O₃ 13.4 13.8 13.9 13.9 12.3 B₂O₃ 5.2 5.2 5.2 5.2 9.1 Li₂O 3.5 3.5 3.7 3.7 3.5 Na₂O 10.2 10.7 10.7 10.7 8.9 K₂O 0.5 0.5 0 0.5 0.5 MgO 2.7 3.3 3.0 2.0 0 ZnO 3.5 1.0 1.0 0.5 0.0 TiO₂ 1.0 1.0 1.0 1.0 1.0 Fe₂O₃ 0.75 0.75 0.75 0.75 0.75 SnO₂ 0.2 0.1 0.1 0 0.1 R₂O/Al₂O₃ 1.06 1.07 1.04 1.07 1.05 Soft pt, ° C. 814 794 799 802 HT CTE, ppm/° C. 21.7 27.0 24.8 Opaque to snake light? (0.8 yes yes yes yes yes mm thick; 570° anneal) Color coordinates (CIE F2, 10° diffuse, reflectance), 0.8 mm thick (570° anneal) L 0.20 0.785 0.54 a* −0.50 −0.21 0.06 b* −0.11 −1.08 −0.16 IX, 430°-15 hr 100 KNO₃, 35 36 DOL (um)

Referring now to Table 9, a series of glass compositions were formulated to assess the effects of the alkali (R₂O) to alumina (Al₂O₃) ratio on forming black glasses. As shown in Table 9, 6 glass compositions were formulated with increasing R₂O:Al₂O₃ ratios. The glass compositions were formed into 0.8 mm glass plates, annealed at 570° C. for 2 hours, cooled to room temperature, and heat treated at 640° C. for 4 hours. Comparative compositions C9 and C10 did not result in an opaque glass using the qualitative assessment described above. Moreover, the softening points of these samples were much greater than 810° C. Comparative composition C12 did not result in an opaque glass using the qualitative assessment described above and the HT CTE of the sample was greater than 27×10⁻⁶/° C. While comparative composition C11 resulted in an opaque glass, the HT CTE of the sample was greater than 27×10⁻⁶/° C. Inventive composition 43 and 44 both yielded opaque glasses using the qualitative assessment described above and both had HT CTEs less than 27×10⁻⁶/° C. The softening point of inventive composition 43 was measured at 812° C. which is approximately 810° C. given the measurement error (+/−5° C.) of the instrument.

TABLE 9 High Boron Glass Compositions With Varying R₂O:Al₂O₃ Ratios (Mol %) C9 C10 43 44 C11 C12 SiO₂ 63.5 63.5 63.5 63.5 63.5 63.5 Al₂O₃ 14.7 14.4 14.1 13.85 13.6 13.2 B₂O₃ 5.2 5.2 5.2 5.2 5.2 5.2 Li₂O 3.5 3.6 3.7 3.7 3.8 3.9 Na₂O 10.2 10.4 10.6 10.8 10.9 11.2 K₂O 0.4 0.4 0.4 0.45 0.5 0.5 MgO 2.0 2.0 2.0 2.0 2.0 2.0 ZnO 0.5 0.5 0.5 0.5 0.5 0.5 TiO₂ 1.0 1.0 1.0 1.0 1.0 1.0 Fe₂O₃ 0.75 0.75 0.75 0.75 0.75 0.75 R₂O/Al₂O₃ 0.96 1.00 1.04 1.08 1.12 1.18 Soft pt, ° C. 837 824 812 804 794 776 HT CTE. 24.6 25.5 26.5 26.8 27.4 31.1 ppm/° C. Opaque to no no yes yes yes no snake light? (0.8 mm thick)** Color coordinates (CIE F2, 10° diffuse, reflectance), 0.8 mm thick* L 4.41 4.56 4.01 4.25 4.21 7.99 a* −1.21 −1.23 −1.08 −1.19 −1.17 −1.52 b* −4.92 −4.93 −4.54 −4.77 −4.78 −6.14 **(570-2 hr anneal + 640-4 hr heat-treat)

Referring now to Table 10, glass plates formed from composition 41 of Table 8 were prepared and heat treated under different conditions. Some of these samples were also ion exchanged to assess the benefit of strengthening glass formed from a glass composition containing color modifying constituents. Specifically, a first set of 17 plates formed from inventive composition 41 and having a thickness of 0.83 mm were annealed at 570° C. for 2 hours. A second set of 17 plates formed from inventive composition 41 and having a thickness of 0.83 mm were annealed at 570° C. for 2 hours, cooled to room temperature, and then ion exchanged in a salt bath of 100% KNO₃ at 440° C. for 15 hours. A third set of 12 plates formed from inventive composition 41 and having a thickness of 0.80 mm were annealed at 570° C. for 2 hours, cooled to room temperature, and then heat treated at 640° C. for 4 hours. A fourth set of 15 plates formed from inventive composition 41 and having a thickness of 0.80 mm were annealed at 570° C. for 2 hours, cooled to room temperature, heat treated at 640° C. for 4 hours, and then ion exchanged in a salt bath of 100% KNO₃ at 440° C. for 15 hours. The concentration of sodium and potassium ions for one plate as a function of depth into the plate is graphically depicted in FIG. 7 indicating that the depth of layer induced by ion exchange was approximately 30 μm. Each of the plates was tested in un-abraded condition using a ring-on-ring testing protocol according to ASTM Standard C1499 in order to assess the degree of strengthening achieved by ion exchange. For purposes of comparison, 15 glass plates formed from Corning glass code 2318 (ion exchange strengthened borosilicate glass sold by Corning Inc. under the trademark Gorilla Glass™) were tested according to the same testing protocol. A Weibull distribution for each set of glass plates was constructed and the characteristic strength and Weibull modulus were determined.

As shown in Table 10, the inventive glass composition 41 had a significant increase in strength following ion exchange in both the annealed condition and the annealed and heat treated condition. This resulted in a degree of strengthening similar to that of found in Corning glass code 2318.

TABLE 10 Ring-On-Ring Testing Strength At Failure Number of Strength at Failure Characteristic Weibull Treatment Thickness (mm) Samples (Mean ± 1 S.D. (% CV)) Strength (So) modulus (m) Annealed (570-2 h), NIX 0.83 17 412 ± 62 (14.9%) 438 7.5 Annealed (570-2 h), 0.83 17 1852 ± 146 (7.9%) 1920 13.8 IX (440-15 h, 100 KNO3) Annealed (570-2 h), 0.80 12 317 ± 85 (26.8%) 352 3.7 Heat-treated (640-4 h), NIX Annealed (570-2 h), 0.80 15 1763 ± 151 (8.6%) 1832 12.5 Heat-treated (640-4 h), IX (440-15 h, 100 KNO3) Control - 2318 IX 1.00 15 1390 ± 320 (23.0%) 1518 4.1

It should now be understood that the glass compositions described herein are suitable for use in conjunction with elevated temperature forming processes for shaping the glass compositions into 3-D shaped glass articles. Specifically, the relatively low softening points of the glass compositions described herein (i.e., softening points of less than or equal to about 810° C.) decrease the interaction between the mold and the glass composition during elevated temperature shaping thereby improving the formability of the glass composition and also increasing the service life of the corresponding mold.

Further, the glass compositions described herein also exhibit a relatively low high temperature coefficient of thermal expansion above the glass transition temperature (i.e., an HT CTE of less than or equal to about 27×10⁻⁶/° C.). The relatively low HT CTE provides the glass compositions good dimensional control of the glass composition following elevated temperature forming processes.

While the glasses described herein have relatively low softening points and relatively low HT CTEs, the glass compositions are also ion exchangeable. For example, the glass compositions described herein may be ion exchange strengthened to achieve a depth of layer of greater than or equal to about 25 μm and a compressive stress of about 650 MPa following immersion in a molten salt bath comprising KNO₃ at about 410° C. for less than or equal to approximately 15 hours.

Further, the glass compositions described herein have liquidus viscosities and viscosities lower than about 200 P at about 1620° C. such that the glass compositions are compatible with fusion draw processes and are easily melted.

It should now be understood that several aspects of glass articles and glass compositions are disclosed herein. In a first aspect, a glass article comprising SiO₂, Al₂O₃, Li₂O and Na₂O has a softening point less than or equal to about 810° C.; a high temperature CTE less than or equal to about 27×10⁻⁶/° C.; and a compressive stress greater than or equal to about 600 MPa and a depth of layer greater than or equal to about 25 μm after ion exchange in a salt bath comprising KNO₃ in a temperature range from about 390° C. to about 450° C. for less than or equal to approximately 15 hours.

In a second aspect, the glass article of the first aspect has L, a*, b*, color coordinates of L from about 0 to about 5.0, a* from about −2.0 to about 2.0, and b* from about 0 to about −5.0.

In a third aspect, the glass article of any of the first or second aspects has an opacity greater than or equal to about 80% over a range of wavelengths from about 350 nm to about 750 nm.

In a fourth aspect, the glass article of any of the first through third aspects comprises from about 65 mol. % to about 71 mol. % SiO₂; from about 7 mol. % to about 12 mol. % Al₂O₃; from about 1 mol. % to about 9 mol. % Li₂O; from about 6 mol. % to about 16 mol. % Na₂O; from about 0 mol. % to about 5 mol. % K₂O; from about 0.8 to about 10 mol. % of a divalent oxide, wherein the divalent oxide comprises at least one of MgO and ZnO; and less than about 0.5 mol. % B₂O₃, wherein a sum of a concentration of Al₂O₃ (mol. %), and a concentration of the divalent oxide (mol. %) is greater than about 10 mol %.

In a fifth aspect, the glass article of the fourth aspect further comprises from about 0.5 mol. % to about 2.0 mol. % ZrO₂.

In a sixth aspect, the glass article of any of the fourth or fifth aspects comprises from about 0 mol. % to about 3 mol. % P₂O₅.

In a seventh aspect, the glass article of any of the fourth through sixth aspects is substantially free of ZrO₂.

In an eighth aspect, the glass article of the seventh aspect comprises from about 65.8 mol. % to about 71 mol. % SiO₂.

In a ninth aspect, the glass article of any of the first through third aspects comprises from about 55 mol. % to about 68 mol. % SiO₂; from about 9 mol. % to about 15 mol. % Al₂O₃; from about 4.5 mol % to about 12 mol. % B₂O₃; from about 1 mol. % to about 7 mol. % Li₂O; from about 3 mol. % to about 12 mol. % Na₂O; and from about 0 mol. % to about 3 mol % K₂O. In this ninth aspect, R₂O is a sum of a concentration of Li₂O, a concentration of Na₂O, and a concentration of K₂O, and a ratio of R₂O to a concentration of Al₂O₃ is less than or equal to about 1.15.

A tenth aspect includes the glass article of the ninth aspect wherein the ratio of R₂O to the concentration of Al₂O₃ is greater than or equal to about 1.02.

In an eleventh aspect, the glass article of any of the ninth or tenth aspects further comprises TiO₂ and Fe₂O₃.

A twelfth aspect includes the glass article of the eleventh aspect, wherein a ratio of Fe₂O₃ (mol. %) to TiO₂ (mol. %) is greater than or equal to about 0.52 and less than or equal to about 1.22.

In a thirteenth aspect, a glass composition includes from about 65.8 mol. % to about 71 mol. % SiO₂; from about 7 mol. % to about 12 mol. % Al₂O₃; from about 1 mol. % to about 9 mol. % Li₂O; from about 6 mol. % to about 16 mol. % Na₂O; from about 0.8 to about 10 mol. % of a divalent oxide, wherein the divalent oxide is at least one of MgO and ZnO; and less than about 0.5 mol. % B₂O₃. A sum of a concentration of Al₂O₃ (mol. %) and a concentration of the divalent oxide (mol. %) is greater than about 10 mol %. The glass composition has a softening point less than or equal to about 810° C. The glass composition also has a high temperature CTE less than or equal to about 27×10⁻⁶/° C. This glass composition may be substantially free from ZrO₂.

A fourteenth aspect includes the glass composition of the thirteenth aspect, wherein the glass composition is substantially free of B₂O₃.

In a fifteenth aspect, the glass composition of any of the thirteenth through fourteenth aspects has a high temperature CTE is less than or equal to about 25×10⁻⁶/° C.

In a sixteenth aspect, the glass composition of any of the thirteenth through fourteenth aspects further includes from about 0 mol. % to about 7 mol. % MgO; from about 0 mol. % to about 1 mol. % CaO; and from about 0 mol. % to about 6 mol. % ZnO.

In a seventeenth aspect, the glass composition of the sixteenth aspect has a concentration of MgO is greater than or equal to about 3 mol. % and less than or equal to about 5 mol. %.

In an eighteenth aspect, the glass composition of any of the thirteenth through seventeenth aspects includes a concentration of SiO₂ from about 68 mol. % to about 71 mol. %.

In a nineteenth aspect, a glass composition includes from about 55 mol. % to about 68 mol. % SiO₂; from about 9 mol. % to about 15 mol. % Al₂O₃; from about 4.5 mol % to about 12 mol. % B₂O₃; from about 1 mol. % to about 7 mol. % Li₂O; from about 3 mol. % to about 12 mol. % Na₂O; and from about 0 mol. % to about 3 mol % K₂O. In this aspect, R₂O is a sum of a concentration of Li₂O, a concentration of Na₂O, and a concentration of K₂O, and a ratio of R₂O to a concentration of Al₂O₃ is less than or equal to about 1.15. The glass composition may have a softening point less than or equal to about 810° . The glass composition may also have a high temperature CTE less than or equal to about 27×10⁻⁶/° C.

A twentieth aspect includes the glass composition of the nineteenth aspect, wherein the softening point is less than or equal to about 800° C.

A twenty-first aspect includes the glass composition of any of the nineteenth through twentieth aspects wherein the high temperature CTE is less than or equal to about 25×10⁻⁶/° C.

A twenty-second aspect includes the glass composition of any of the nineteenth through twentieth aspects wherein the ratio of R₂O to the concentration of Al₂O₃ is greater than or equal to about 1.02 and the glass composition further comprises TiO₂ and Fe₂O₃.

A twenty-third aspect includes the glass composition of any of the nineteenth through twenty-second aspects wherein a ratio of Fe₂O₃ (mol. %) to TiO₂ (mol. %) is greater than or equal to about 0.52 and less than or equal to about 1.22.

A twenty-fourth aspect includes the glass composition of any of the nineteenth through twenty-third aspects, wherein the glass composition further comprises from about 0 mol. % to about 5 mol. % MgO; from about 0 mol. % to about 5 mol. % ZnO; and from about 0 mol. % to about 2 mol. % CaO.

A twenty-fifth aspect includes the glass composition of any of the nineteenth through twenty-fourth aspects, wherein a concentration of B₂O₃ is greater than or equal to 7 mol. % and less than or equal to 12 mol. %.

A twenty-sixth aspect includes the glass composition of any of the nineteenth through twenty-fifth aspects, wherein the glass composition comprises from about 0 mol. % to about 3 mol. % P₂O₅.

A twenty-seventh aspect includes a glass composition comprising from about 65 mol. % to about 71 mol. % SiO₂; from about 7 mol. % to about 12 mol. % Al₂O₃; from about 1 mol. % to about 9 mol. % Li₂O; from about 6 mol. % to about 16 mol. % Na₂O; from about 0 mol. % to about 5 mol. % K₂O; from about 0.8 to about 10 mol. % of a divalent oxide, wherein the divalent oxide comprises at least one of MgO and ZnO; from about 0 mol. % to about 3 mol. % P₂O₅, from about 0.5 mol. % to about 2 mol. % ZrO₂; and less than about 0.5 mol. % B₂O₃. In this aspect, a sum of a concentration of Al₂O₃ (mol. %), and a concentration of the divalent oxide (mol. %) is greater than about 10 mol %. The glass composition may have a softening point less than or equal to about 810° C. The glass composition may also have a high temperature CTE less than or equal to about 27×10⁻⁶/° C.

It will be apparent to those skilled in the art that various modifications and variations can be made to the embodiments described herein without departing from the spirit and scope of the claimed subject matter. Thus it is intended that the specification cover the modifications and variations of the various embodiments described herein provided such modification and variations come within the scope of the appended claims and their equivalents. 

1. A glass article comprising SiO₂, Al₂O₃, Li₂O and Na₂O, the glass article having: a softening point less than or equal to about 810° C.; a high temperature CTE less than or equal to about 27×10⁻⁶/° C.; and a compressive stress greater than or equal to about 600 MPa and a depth of layer greater than or equal to about 25 μm after ion exchange in a salt bath comprising KNO₃ in a temperature range from about 390° C. to about 450° C. for less than or equal to approximately 15 hours. 2-8. (canceled)
 9. The glass article of claim 1 comprising: from about 55 mol. % to about 68 mol. % SiO₂; from about 9 mol. % to about 15 mol. % Al₂O₃; from about 4.5 mol % to about 12 mol. % B₂O₃; from about 1 mol. % to about 7 mol. % Li₂O; from about 3 mol. % to about 12 mol. % Na₂O; and from about 0 mol. % to about 3 mol % K₂O, wherein: R₂O is a sum of a concentration of Li₂O, a concentration of Na₂O, and a concentration of K₂O, and a ratio of R₂O to a concentration of Al₂O₃ is less than or equal to about 1.15.
 10. The glass article of claim 9, wherein the ratio of R₂O to the concentration of Al₂O₃ is greater than or equal to about 1.02.
 11. The glass article of claim 10, wherein the glass article further comprises TiO₂ and Fe₂O₃.
 12. The glass article of claim 11, wherein a ratio of Fe₂O₃ (mol. %) to TiO₂ (mol. %) is greater than or equal to about 0.52 and less than or equal to about 1.22. 13-18. (canceled)
 19. A glass composition comprising: from about 55 mol. % to about 68 mol. % SiO_(2;) from about 9 mol. % to about 15 mol. % Al₂O₃; from about 4.5 mol % to about 12 mol. % B₂O₃; from about 1 mol. % to about 7 mol. % Li₂O; from about 3 mol. % to about 12 mol. % Na₂O; and from about 0 mol. % to about 3 mol % K₂O, wherein: R₂O is a sum of a concentration of Li₂O, a concentration of Na₂O, and a concentration of K₂O, and a ratio of R₂O to a concentration of Al₂O₃ is less than or equal to about 1.15; the glass composition has a softening point less than or equal to about 810° C.; and the glass composition has a high temperature CTE less than or equal to about 27×10⁻⁶/° C.
 20. The glass composition of claim 19, wherein the softening point is less than or equal to about 800° C.
 21. The glass composition of claim 19, wherein the high temperature CTE is less than or equal to about 25×10⁻⁶/° C.
 22. The glass composition of claim 19, wherein the ratio of R₂O to the concentration of Al₂O₃ is greater than or equal to about 1.02 and the glass composition further comprises TiO₂ and Fe₂O₃.
 23. The glass composition of claim 22, wherein a ratio of Fe₂O₃ (mol. %) to TiO₂ (mol. %) is greater than or equal to about 0.52 and less than or equal to about 1.22.
 24. The glass composition of claim 19, further comprising: from about 0 mol. % to about 5 mol. % MgO; from about 0 mol. % to about 5 mol. % ZnO; and from about 0 mol. % to about 2 mol. % CaO.
 25. The glass composition of claim 19, wherein a concentration of B₂O₃ is greater than or equal to 7 mol. % and less than or equal to 12 mol. %.
 26. The glass composition of claim 19, wherein the glass composition comprises from about 0 mol. % to about 3 mol. % P₂O₅.
 27. (canceled) 